1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an engine, and more specifically to a technology for purifying nitrogen oxide discharged from an engine using ammonia as a reducing agent.
2. Description of the Related Art
As an apparatus for purifying air pollutant discharged from an engine, particularly, nitrogen oxide (hereinafter referred to as “NOx”) in the exhaust gas, by post treatment, a following SCR (Selective Catalytic Reduction) unit has been known. That is, a device for injecting ammonia or its precursor aqueous solution into an exhaust gas passage of the engine and with the injected ammonia used as a reducing agent, NOx and this ammonia are allowed to react with each other on a catalyst so as to reduce and purify NOx. Further, there has been also known an SCR unit in which, considering easiness of storage of ammonia on a vehicle, urea as a precursor of the ammonia is stored in a tank in the state of aqueous solution, and urea aqueous solution supplied from this tank is injected into the exhaust gas passage so as to generate ammonia by hydrolysis of urea using exhaust heat (Japanese Laid-Open (Kokai) Patent Application Publication No, 2000-027627 (paragraph number 0013)).
The inventor of the present patent application has contrived adopting of this SCR unit to the exhaust gas purifying apparatus of a vehicular engine and has already succeeded in actual application. It is important to provide a urea sensor in a tank capable of storing therein the urea aqueous solution (hereinafter, referred to as urea aqueous solution tank), in order to perform reduction action of NOx favorably by injecting an appropriate amount of urea aqueous solution to NOx discharge amount from the engine, and reflect an actual concentration of urea (hereinafter when just “concentration” is mentioned, the concentration of urea shall be referred to) on control operation of the engine and the SCR unit. Currently, as a urea sensor, there has been developed a type in which a heater and a temperature measuring resistor are equipped with and an actual concentration is determined based on an electric resistance value obtained by the temperature measuring resistor heated by the heater, by paying attention to the heat transmission characteristic of urea aqueous solution depending on the concentration (Japanese Laid-Open (Kokai) Patent Application No. 2005-030888 (paragraph number 0044)). This inventor has disclosed an exhaust gas purifying apparatus for an engine which adopts a temperature-sensitive type urea sensor in Japanese Patent Application No. 2003-366737 (Japanese Laid-Open (Kokai) Patent Application Publication No. 2005-133541). According to this apparatus, when the concentration detected by a urea sensor is lower than the normal range of concentration, it is estimated that water or other aqueous solution than urea aqueous solution of a predetermined concentration is stored in the urea aqueous solution tank, and then a further detection is performed to become aware of an abnormality about the concentration and some measures such as stopping of injection of urea aqueous solution is resultantly taken.
Here, concerning the exhaust gas purifying apparatus for the engine adopting the temperature-sensitive urea sensor, following problems are encountered. In such an apparatus that a tank stores a urea in the state of aqueous solution, a tank heater is provided in order to prevent urea aqueous solution from being frozen within the urea aqueous solution tank or unfreeze the frozen urea aqueous solution quickly, particularly, on an assumption that the apparatus is used in a cold district (Japanese Laid-Open (Kokai) Patent Application Publication No. 2004-194028). When a temperature-sensitive type urea sensor is adopted, convection flow of the urea aqueous solution is generated within the urea aqueous solution tank accompanied by heating of this tank heater, thereby adversely affecting detection of the concentration performed by the urea sensor. Due to unevenness in temperature of the urea aqueous solution caused by this convection flow, the concentration detected by the urea sensor is dispersed, so that detection accuracy of the concentration might drop, thereby causing errors in detection of an abnormality with the detected concentration used as basic information.
Accordingly, upon adopting the temperature-sensitive type concentration sensor, it is demanded to have a capability of avoiding erroneous detection of abnormality originating from convection of aqueous solution in a storage tank and prevent malfunction of the exhaust gas purifying apparatus, such as unexpected stopping of addition of a reducing agent.